Charge coupled devices (CCDs), complementary metal oxide semiconductor (CMOS) devices, and infrared imagers, which may be referred to generally as Solid State Area Array Imaging Devices (SSAAIDs), are used to capture images received in the form of light. They are currently widely used for both defense and commercial purposes. Some popular uses include digital cameras, scanners, cell phones, and surveillance devices.
SSAAIDs contain pixels arranged in a grid, which is referred to as a Focal Plane Array (FPA). Each pixel of an SSAAID generates and holds an amount of charge proportionate to the intensity of light incident thereon and the length of time that light was allowed to fall on the pixel using an integration circuit.
An integration circuit performs the mathematical operation of integration with respect to time. Said another way, the output voltage of an integration circuit is proportional to the input voltage, integrated over time (Output∝∫Input). In the case of a pixel, the input voltage is generated by the impact of photons on a detector. The charge handling capacity of such a circuit is determined by voltage, integration time, and capacitance of its capacitor(s).
Current SSAAIDs are limited in their ability to provide acceptable images in moderate to low light level conditions as well as in high light level conditions by the dynamic range of the integration circuit of the pixels. In low light level conditions, where there are relatively few incoming photons incident on any given pixel, the signal-to-noise ratio (SNR) of the output is very low, resulting in a grainy/noisy image in dark areas of the image. Moreover, in low SNR situations, other variables can also create non-uniformities in the images where the signal levels are not sufficient to overcome the sensitivity anomalies.
Pixel integration circuits may also become saturated in high light level conditions. When a large amount of light hits a pixel, the integration circuit of that pixel, and even those of nearby pixels due to a phenomenon referred to as “blooming”, become saturated, a situation that results in the integration circuit ceasing to be able to capture additional information. Saturation results in washed out images or portions thereof. Although anti-blooming circuits may be used to help reduce the impact of one or a cluster of saturated pixels on others, to increase high light level performance of a given pixel requires increasing the capacity, or well size, of its integration circuit, thereby preventing saturation over a given interval of time.
Prior art FPAs have used shorter integration times to provide better low gain, or high light, performance, but are less sensitive as a result and therefore less able to capture low light level conditions.
Integration circuits may beneficially include compact signal averaging circuits, such as “Compact Signal Averager” (CSA) circuit invented by Mr. Dan Lacroix on which a patent was filed by Loral Infrared & Imaging Systems on May 9, 1994 (see U.S. Pat. No. 5,448,189). The CSA circuit has been used in various applications with circuit configurations as documented in the referenced patent.
Among other applications, the CSA is often used in applications where short-duration, transient noise is a significant issue. One such application is radiation exposure, especially exposure to transient gamma radiation, which causes some subframes to read much higher than others. CSA suppresses that noise since, by design, it only accepts a small portion of the additional high signal. Even for very large prompt pulse events that contaminate the average storage, once the radiation pulse dissipates and one clean subframe is recorded, the circuit can recover quickly.
While CSA excels at short, strong signal suppression, there exists a concern that CSA circuits might suppress valid transient signals. Switched capacitor filter circuits, which do not suppress transients, are more desirable in this case.
Existing CSA circuits, however, are not compatible with switched capacitor filter circuits, which in addition to the benefit described above, also allow for benefits in dynamic range. Furthermore, existing CSA circuits cannot fit on a single pixel with a switched capacitor filter circuit. Existing CSA circuits also have very limited dynamic range.
What is needed, therefore, is a CSA circuit that is compatible with switched capacitor filter circuits, offers improved dynamic range, allows for switching between the two, and can fit on a single pixel with a switched capacitor filter circuit.